Field
This invention relates to cyanoacrylate-containing compositions, which when cured provide improved heat resistance.
Brief Description of Related Technology
Cyanoacrylate adhesive compositions are well known, and widely used as quick setting, instant adhesives with a wide variety of uses. See H. V. Coover, D. W. Dreifus and J. T. O'Connor, “Cyanoacrylate Adhesives” in Handbook of Adhesives, 27, 463-77, I. Skeist, ed., Van Nostrand Reinhold, New York, 3rd ed. (1990). See also G. H. Millet, “Cyanoacrylate Adhesives” in Structural Adhesives: Chemistry and Technology, S. R. Hartshorn, ed., Plenun Press, New York, p. 249-307 (1986).
In the past, efforts have been made to improve the heat resistance of cured products of cyanoacrylate composition, particularly upon exposure to temperatures of 80° C. and greater. As the cured products are thermoplastic in nature they tend to soften as the temperature increases and when the Tg of the material is surpassed the cured product begins to flow. As the temperature increase progresses, a degradation begins and the physical properties deteriorate. As a result, commercial applications for cyanoacrylates where exposure to elevated temperature conditions is likely have proven tricky and consequently have been limited.
Attempts to remedy this situation have been undertaken in the past.
For instance, U.S. Pat. No. 5,288,794 (Attarwala) is directed to an improved cyanoacrylate monomer adhesive formulation, where an effective amount, for enhancing the thermal resistance of the polymerized adhesive, of a mono, poly or hetero aromatic compound characterized by at least three substitutions on an aromatic ring thereof, two or more of the substitutions being electron withdrawing groups, is provided. Examples of the aromatic compound are given as 2,4-dinitrofluorobenzene; 2,4-dinitrochlorobenzene; 2,4-difluoronitrobenzene; 3,5-dinitrobenzonitrile; 2-chloro-3,5-dinitrobenzonitrile; 4,4′-difluoro-3,3′-dinitrophenyl sulfone; pentafluoronitrobenzone; pentafluorobenzonitrile; α,α,α-2-tetrafluoro-p-tolunitrile and tetrachloroterphthalonitrile.
Prior to the discovery in the '794 patent, numerous attempts have been made to improve the thermal stability of cyanoacrylate adhesive bonds.
For instance, U.S. Pat. No. 3,832,334 is directed to the addition of maleic anhydride, which is reported to produce cyanoacrylate adhesives having increased thermal resistance (when cured) while preserving fast cure speed.
U.S. Pat. No. 4,196,271 is directed to tri-, tetra- and higher carboxylic acids or their anhydrides, which are reported to be useful for improving heat resistance of cured cyanoacrylate adhesives.
U.S. Pat. No. 4,450,265 is directed to the use of phthalic anhydride to improve heat resistance of cyanoacrylate adhesives. More specifically, the '265 patent is directed to and claims an adhesive composition comprising a polymerizable constituent the major part of which comprises at least one ester of 2-cyanoacrylic acid, characterized in that the composition additionally comprises a proportion of phthalic anhydride effective for favorably influencing the strength and/or durability of adhesive bonds formed from the composition, under exposure to moisture or elevated temperature. The effective amount is reported as 0.1% to 5.0%, such as 0.3% to 0.7%, by weight of the composition. The '265 patent reports the superiority of phthalic anhydride over compositions where no additive was used, and where maleic anhydride was used (though less pronounced in the case of stainless steel lap shears than in those of aluminium).
U.S. Pat. No. 4,532,293 is directed to the use of benzophenonetetracarboxylic acid or its anhydride to provide a superior heat resistance for cyanoacrylate adhesives.
U.S. Pat. No. 4,490,515 is directed to cyanoacrylate compositions containing certain maleimide or nadimide compounds to improve hot strength properties.
U.S. Pat. No. 4,560,723 describes certain cyanoacrylate adhesives containing a certain treated copolymer toughener, and a “sustainer” compound having certain compatibility properties said to provide improved retention of toughness on heat aging of the cured adhesive. Various substituted aryl compounds are identified as suitable “sustainers,” including nitrobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene and bromochlorobenzene.
One way to improve thermal stability without resorting to additive chemistry involves the use of an allyl-2-cyanoacrylate, in whole or in part, as the cyanoacrylate component. It is believed that allyl-2-cyanoacrylate will undergo a cross-linking reaction through the allyl functional groups, once given a post-bake (either as an additional process step or as a result of the environment in which they are used) after the initial cure occurs.
While the application of a post-bake certainly provides an allyl-2-cyanoacrylate-containing adhesive with significant improvements in thermal durability (see FIG. 1), post-bake processing is not devoid of its own issues. For instance, the additional step of a post-bake requires added labor, time and expense to the process. And, a post-bake results in a considerable loss in initial performance while the cross-links are being formed thermally. This loss is referred to as “the dip”, with bond strength recovery occurring after approximately one week at a temperature of 150° C. (See FIG. 1.)
Despite these efforts, there has been a long standing, but unmet, desire to achieve more robust thermal performance from cyanoacrylate compositions. It would accordingly be quite advantageous to provide a solution to that desire.